This is because both Chrome and Firefox are nearing version 100 and because of this, the way websites identify browsers could cause it to run into issues when either browser is updated to version 100.
According to a post on Mozilla’s website, “Major version 100 is a big milestone for both Chrome and Firefox. It also has the potential to cause breakage on websites as we move from a two-digit to a three-digit version number.”
The post adds, “Without a single specification to follow, different browsers have different formats for the User-Agent string, and site-specific User-Agent parsing. It’s possible that some parsing libraries may have hard-coded assumptions or bugs that don’t take into account three-digit major version numbers.”
The good news is that both Google and Mozilla have been working to fix this issue, one of which involves tricking websites into thinking the current version is version 100 and then identifying the issues and then fixing them. Both companies also have plans to potentially freeze the major version at 99 until a more permanent fix can be found.